1986 244 DL, 225K miles. On cold start,the engine stumbles if engine is revved, and may choke and die even on idle. After 5 to 10 minutes, engine smooths out, and will no longer choke on revving.

Diagnostics: Spark is good on all cylinders. Injector delivers on all cylinders. MAF working. Oxy sensor working. When engine is still cold, if I remove the vacuum hose from the fuel pressure regulator, the engine perks up and no longer stumbles.

Diagnostics not done: Cannot do fuel pressure checks (no fittings for gauge).

New or recent parts: New air filter, new cap and rotor. Recent fuel pumps, fuel filter, fuel pump relay (all within the past year).

My initial thought was the fuel pressure regulator. But I'd like to know what ELSE it might be, given the conditions described above. Thanks.

 

gummed up throttle body or idle air control valve, or the idle microswitch isn't engaging/adjusted properly?